You shouldn't say something harsh to somebody anyway, but thinking that question in your head might prevent that. Everyone is fighting a battle, and many people are skilled in hiding it. If someone is "use to" dealing with pain, you might not be able to tell that there is something wrong, but that doesn't mean they aren't dying on the inside.
Try to be positive and kind to everyone you encounter. Sometimes a person just needs one compliment, one piece of advice, or even just a smile. It costs absolutely nothing to be nice. I know this may sound repetitive, but I can't stress this issue enough. If you speak rudely to a person, and then find out afterwards that maybe a tragedy has occurred recently in their life, that only leaves the both of you feeling terrible. And sometimes, "sorry" doesn't mean much. You can't take back anything you said.
The phrase "sticks and stones may break my bones, but words will never hurt me" is a lie. Broken bones can be healed, and eventually be like new again. But someone who's been bullied, will be a changed person forever. And a lot of the time, they've changed for the worst. Their self esteem, body image, trust, and perception is demolished.
Your words can break someone, or make them. What will your words do?